This invention relates to a process for the reduction of sulfur dioxide and to a method of separating sulfur dioxide from a gaseous mixture containing the same wherein the sulfur dioxide is subsequently reduced with the reducing agent of this invention. More particularly, this invention relates to a process for the reduction of sulfur dioxide with ammonia and to a method of separating sulfur dioxide from a gaseous mixture containing the same wherein the separated sulfur dioxide is subsequently reduced with ammonia.
The reduction of sulfur dioxide generally, and the reduction of sulfur dioxide, specifically, in a regeneration off gas stream from a flue gas desulfurization process are, of course, known. For example, F. M. Dautzenberg et al, Chemical Engineering Progress, 67, No. 8, pp. 86-91 (August 1971) describes a process for desulfurizing flue gas using a solid copper oxide on alumina sorbent, regenerating the sorbent with a reducing gas such as hydrogen, and reducing the sulfur dioxide in the regeneration off gas to elemental sulfur. The off gas may be concentrated to 100% SO.sub.2 prior to reduction if desired. The stream of sulfur dioxide or regeneration off gas is split into two portions containing 2/3 and 1/3, respectively, of the total; the larger portion is reduced catalytically to hydrogen sulfide by hydrogen or other reducing gas, and the hydrogen sulfide is catalytically reacted with the smaller portion of sulfur dioxide to form elemental sulfur. Similarly, U.S. Pat. No. 3,495,941 discloses a process in which sulfur dioxide in regeneration off gas is catalytically reduced to hydrogen sulfide with hydrogen, a hydrogen-containing gas, or a hydrocarbon such as methane. The hydrogen sulfide can be reacted with sulfur dioxide to form elemental sulfur. U.S. Pat. No. 3,630,943 discloses the reduction of sulfur dioxide in a regeneration off gas stream to hydrogen sulfide and sulfur in a Claus plant.
Processes for reducing sulfur dioxide from other sources to elemental sulfur with other reducing agents are also known. In this regard, it should be noted that U.S. Pat. No. 2,148,258 describes a process in which part of the sulfur dioxide content of a gas obtained by acid sludge decomposition is reduced either catalytically or non-catalytically at temperatures not over 1200.degree. F. to hydrogen sulfide and elemental sulfur, using a solid, liquid or gaseous reducing agent (e.g., carbon, hydrocarbons, or hydrogen) followed by catalytic conversion of hydrogen sulfide and unreacted sulfur dioxide to elemental sulfur. Similarly, U.S. Pat. Nos. 2,388,259 and 2,431,236 disclose processes in which sulfur dioxide in smelter gas (which also contains oxygen) is thermally reduced with natural gas, followed by passage of the thermal reactor effluent through a pair of catalytic converters in series to convert hydrogen sulfide and sulfur dioxide to elemental sulfur. U.S. Pat. No. 2,270,427 describes a system in which sulfur dioxide and added air are thermally reduced with methane, and in which the thermal reactor effluent is passed through a first stage COS coversion catalyst and then through a second stage H.sub. 2 S conversion catalyst, the second catalyst stage being operated below the dew point of elemental sulfur. U.S. Pat. No. 1,917,685 discloses a process comprising reducing sulfur dioxide in smelter gases with a carbonaceous reducing agent (e.g., water gas, producer gas, natural gas, or powdered coal) to elemental sulfur and hydrogen sulfide, quickly cooling the exit gas stream, condensing elemental sulfur therefrom, and catalytically converting H.sub.2 S, COS and SO.sub.2 in the exit gas to form elemental sulfur.
U.S. Pat. No. 1,741,551 describes a process in which a gas containing SO.sub.2 (e.g., smelter gas) is passed through a bed of incandescent solid carbonaceous fuel, the resulting gas mixture is reacted with additional SO.sub.2 in a combustion chamber to form a gas mixture containing sulfur vapor, and any H.sub.2 S and SO.sub.2 in the combustion chamber effluent gas is catalytically converted to elemental sulfur by reaction in a "hot" catalyst bed at temperatures above 300.degree. C. followed by reaction in a "cold" catalyst bed at 125.degree.-200.degree. C. In the process disclosed in U.S. Pat. No. 1,773,294, an SO.sub.2 -containing gas is reduced with an incandescent carbonaceous fuel followed by reaction of the resulting gas mixture with additional SO.sub.2 in a combustion chamber. The combustion chamber effluent is either catalytically treated at 280.degree.-350.degree. C., or cooled to condense elemental sulfur and catalytically treated at about 120.degree.-200.degree. C. T. F. Doumani et al, Industrial and Engineering Chemistry, Vol. 36, No. 4, pp. 329-332 (April 1944) discloses a process in which sulfur dioxide in a waste gas stream is catalytically reduced to sulfur and hydrogen sulfide; the latter is reacted with additional sulfur dioxide to form elemental sulfur.
As a general proposition, it must be concluded that the prior art reducing agents are, basically, effective and that, indeed, any of the prior art reducing agents could be used to convert sulfur dioxide to hydrogen sulfide, elemental sulfur or to a mixture of both. The use of a carbonaceous reducing agent is, however, accompanied with an undesirable tendency to form soot, carbon oxysulfide and carbon disulfide as biproducts of the reducing reaction. These products are, of course, undesirable in that all will, to some extent, adversely affect the purity of the more desirable sulfur product or products. Moreover, soot will, generally, discolor the elemental sulfur product while the carbon oxysulfide and carbon disulfide, both of which are not easily converted to elemental sulfur, will decrease the yield thereof. Also, since the more preferred of the prior art reducing agents are gases, availability at the site and storage have created some problems with respect to use thereof.
Processes for the separation of sulfur dioxide from various gaseous mixture comprising the same are also well known in the prior art. Generally, these prior art processes involve first contacting the gaseous mixture with an absorbent which combines with the sulfur oxide to form either a solid or liquid and thus separates them from the gaseous mixture, followed by either disposal of the solid or liquid product or regeneration of the absorbent with the release of absorbed sulfur oxide. Such sulfur oxide removal processes have been classified as either "throw-away" processes or regenerative processes. Throw-away processes include, for example, processes in which sulfur oxides in a gas stream are reacted with calcium oxide (or calcium carbonate) and the resultant calcium sulfite and sulfate are discarded. Regenerative processes include both processes using a dry solid sorbent or acceptor, or a liquid (usually aqueous) scrubbing medium. An example of a dry solid sorbent is copper oxide on alumina which is described in British Patent 1,089,716. Other regenerative processes use a liquid and usually aqueous scrubbing medium which is capable of reacting with SO.sub.2 (and SO.sub.3 when present), followed by regeneration of the scrubbing medium. The use of ammonia in an aqueous medium for removing sulfur oxides from gases has been suggested in a number of references, and equilibria in the system SO.sub.2 -NH.sub.3 -H.sub.2 O have been reported in detail by H. F. Johnstone, Industrial & Engineering Chemistry, volume 27, page 587 (1935) and ibid, Volume 30, page 101 (1938). Examples of references describing the scrubbing of an SO.sub.2 -containing gas stream with aqueous ammonia (either as ammonium hydroxide, ammonium sulfite, or ammonium sulfite-bisulfite) with liberation of sulfur dioxide on regeneration, include U.S. Pat. Nos. 2,134,481 (Johnstone), 2,405,747 (Hixson et al), and 3,645,671 (Griffin et al). The sulfur dioxide stream liberated on regeneration of the absorbent solution has a much higher concentration of SO.sub.2 than the original flue gas stream. This sulfur dioxide is then converted to sulfuric acid by known means, although, as indicated previously, processes have been suggested to reduce such SO.sub.2 to H.sub.2 S or elemental sulfur. The spent scrubbing solution in these references is regenerated and reused for scrubbing further quantities of sulfur dioxide containing gas.
It is known that the quantity of sulfur oxides in a gas stream can be reduced by injecting gaseous ammonia into the gas stream. Such processes are disclosed, for example, in Chemical and Engineering News, Sept. 11, 1972, pp. 54 and 56, and in C. C. Shale et al, "Removal of Sulfur and Nitrogen Oxides from Stack Gases by Ammonia," AIChE Chemical Engineering Progress Symposium Series No. 115, Vol. 67, pages 52-58 (1971).
A principal problem associated with the prior art processes, however, is that a multiplicity of treating agents are, generally, required. For example, in some processes one treating agent, e.g. CuO, might be used as the absorbent, a second treating agent; e.g. hydrogen, might be used to regenerate the absorbent, and a third treating agent; e.g. methane, might be used to reduce the sulfur dioxide liberated in the regeneration step. Such multiple use does, of course, further complicate those problems associated with availability at the site as well as those problems associated with storage at the site once the treating agents are there available.
The need, then, for a reducing agent which, when not readily available at the site of intended use, might be conveniently transferred and stored thereat is believed to be readily apparent. Similarly, the need for a process which will, effectively, permit the separation of sulfur dioxide from a gaseous mixture containing the same and the subsequent reduction thereof to either elemental sulfur, hydrogen sulfide or a mixture of both with a single treating agent is also believed to be readily apparent.